Discharge device



March 28, 1939. L. D. MILES 2 DISCHARGE DEVICE 5 Sheets-Sheet 1 FiledOct. 9, 1937 Inventor Lawrence D. Mi l es, 1:: #044 His 63,9 ttorn ey.

March 28, 1939. 1.. 0 MILES DISCHARGE DEVICE Filed on. 9, 1937 5 Sheets-Sheet 2 Ffg. 2

Inventor. Lawrence D. Miles, b W 6. IMAM. His A torney.

March 28, 1939.

' L. D. MILES DISCHARGE DEVI-CE Filed Oct. 9, 1937 5 Sheets-Sheet 3Fig.6.

Inventor Lawrence Miles, b. JV 11w Attorney.

March 28, 1939. L. D.- MILES 2,152,201

' I DISCHARGE DEVICE Filed Oct. 9, 19:57 5 She'ets--Sheet 4 Hg. IO.

March 28, 1939. L, D. MILES I I DISCHARGE DEVICE 5 Sheets-Sheet 5 FiledOct. 9, 1957 Fig. l4.

Miles,

His ttorney.

m t J mmfi n e IF W y b I Patented Mar.' 28, 1939 UNITED STATES2.152.201 nrscmmcn pnvrcn Lawrence D. Miles, Schenectady, N. it,ass'ignor to General Electric company, a corporation oi New YorkApplication October a, 1931, Serial No. 108,191 9 Claims. (in. also-21.5

The present invention relates to improvements in discharge devicesemploying cathodes constituted of a material which is adapted to yieldsubstantially unlimited electron emission in the presence of a favorableelectrostatic field. Such cathodes are best exemplified by the mercurypool and for that reason are referred to in this application aspool-type cathodes". It should be understood, however, that the term"pooltype" as used herein and in the appended claims is not limited toliquid metals such as mercury but is intended also to include solidsubstances which are capable of analogous use. Particular examples ofsuch solid substances include cadmium and bismuth.

It is an object of the invention to provide an improved means forinitiating a discharge between the principal electrodes of a dischargedevice of the class above specified. While not limited thereto, theinvention is primarily applicable to single anode devices or otherdevices in which the main discharge requires to be reinitiated atcyclically recurrent intervals.

It is well known in this connection to utilize means for continuouslymaintaining a holding are from which an intermittent main discharge maybe ignited from time to time. It is, however, an objectionablecharacteristic of such systems as conventionally employed that thecathode spot is free to wander on to the walls of the dischargeenclosure unless such walls are eflectively insulated. It is oneimportant object of this invention to provide a holding-arc assemblywhich may be used satisfactorily even where the metal parts of thedischarge enclosure are electrically continuous with the cathode itself.

This is accomplished in one way by so enclosing or shielding thedischarge space of the holding are that it is substantially entirelysep- -10 arated from the main discharge space except for a restrictedpassage between them. By utilizing shielding means of appropriatecharacter, as hereinafter explained, I obtain the result that thecathode spot of the holding arc is always 45 confined to a restrictedcathode region and always causes the main discharge to develop from suchregion. Consequently, even if a cathode spot attributable to the maindischarge current reaches the enclosure wall during a portion of to somegiven voltage cycle, it will necessarilybe reborn in the vicinity of theholding arc assembly at the beginning of the next cycle.

Another advantage accruing from the use of means for shielding theholding arc lies in the a.) fact that such means effectively protect theholding are from possible extinguishment by arcback occurrences in themain discharge space. This is of considerable practical importance inthat it assures immediate resumption of operation at the end of eacharc-back period and eliminates the need for elaborate supervisoryequipment for automatically restarting the holding are.

It is frequently desirable in connection with pool-type dischargedevices to control or vary o the time of discharge initiation. and afurther aspect of my invention consists in the combination with ashielded holding arc assembly of means for accomplishing such control.

The features of novelty which I desireto pro- 1 tect herein are pointedout with particularity in the ap ded claims. The invention itself,together with further objects and advantages thereof, may best beunderstood with reference to the following description taken inconnection 20 with the drawings in which Fig. 1 illustrates in section apool-type discharge device suitably embodying the invention;.Figs. 2-5are various detailed views of a particular holding arc assemblyaccording to the invention; Figs. 6 and 7 con- 25 jointly illustrate apossible modification of the holding arc assembly; Fig. 8 shows afurther modification Fig. 9 shows a variant means which may be usedalternatively with the make-alive means of Figs. 1-8 for initiallystarting the holdso ing arc; Fig. 10 shows the combination of a shieldedholding arc assemblywith means for controlling the time of initiation ofthe main discharge; Fig. 11 shows circuit connections suitable for usewith the device of Fig. 10 and Figs.

12-16 illustrate two modifications of the controllable device of Fig.10.

Referring particularly to Fig. 1, I have shown a metal envelope. l0forming an enclosed discharge space within which are arranged a cath- 4oode -II and an anode l2, providing appropriate discharge terminals.Asshown, the anode comprises a massive member of heat-resistingmaterial, such as graphite, supported froin the envelope and insulatedfrom it by means of a glass ring II. It should be understood, however,that in certain cases the anode may comprise a portion of the envelopeitself. The cathode II is a pool-type cathode, as -previously defined,and may consist of a liquid metal such as mercury or of an equivalentsolid material such as-cadmium. It is in direct contact with the mainbody of the metal envelope and is thus electrically continuous with it.

Devices such as that illustrated are ordinarily used as asymmetricallyconducting elements in When they are so used, some means must beprovided for facilitating the initiation of a main discharge upon theapplication of favorable potential between the discharge terminals. Ihave shown for this purpose a holding-arc assembly l6 having certainnovel and improved features to be described in the following. Beforeproceeding to a full description of such features, it will be helpful torefer briefly to certain factors involved in the operation of devices ofthe type in question.

An arc discharge extending from a pool-type cathode results in thecreation of a so-called cathode spot on the cathode surface. Actuallysuch a spot comprises a multiplicity of separate spots" or elementswhich grow by subdivision as the current increases until on a liquidmercury surface a minute spot exists for about each two amperes. Thus,600 amperes come from about 300 spots which usually arrange themselvesin further groups and all act in parallel. In a discharge devicecomprising one main anode and holding anode, the cathode spot comprisesa continuously maintained component, attributable to the holding arc,and an intermittent or cyclically recurrent component, attributable tothe main discharge. The latter component grows or develops from theformer in the presence of a favorable potential field in accordance withthe process of subdivision previously referred to. with an open holdingare as conventionaly employed, the composite cathode spot, beingcontinuously maintained although variable in size,

travels rapidly from place to place, moves frequently on to the metaltube walls unless insulated from such walls, and when finally maroonedupon the metal, gets its emission therefrom and disintegrates the metalin so doing. It may wander back to the cathode surface without seriousconsequences or it may wander into the seals and damage them. For thisreason, heretofore, the cathode has been conventionally insulated fromthe envelope side walls when a holding arc is employed.

However, a holding-arc assembly may satisfactorily be employed inconnection with a discharge device in which the metal walls of theenvelope are electrically continuous with the surface of the cathodeprovided the component of the cathode spot attributable to the holdingarc is confined to a restricted region of the cathode surface. If thisis done, the main discharge current will necessarily be initiated at apoint adjacent to such region at the beginning of each voltage cycle.Consequently, no portion of the cathode spot will be able to lodgepermanently or for a substantial length of time on any part of thedischarge device other than the cathode surface.

Confinement of the holding arc cathode spot as proposed in the foregoingis accomplished in accordance with my invention by the use of shieldingmeans adapted substantially to segregate the holding are from the maindischarge space except for the provision of a restricted passage betweenthem. Referring again to Fig. 1, I have shown one example of such ashielding means as comprising a hollow. enclosing member extending fromthe surface of the cathode ll up to and around the keep-alive electrodesl1 and I8. This member is preferably constituted throughout ofrefractory material adapted to withstand the effects of an arcdischarge.

The details of construction of the holding-arc assembly are clearlyillustrated in Figs. 2-5 in which parts corresponding to elements shownin Fig. 1 are similarly numbered. It is possible, of course, to mountthe assembly in various ways as by supporting it from the top or lateralwalls of the discharge envelope. However, I prefer an arrangement suchas that shown, wherein the assembly as a whole is mounted on ademountable header (Fig. 2) which is positioned within a conformingcylinder 22 depending from the bottom of the envelope. The header issecured in place by an externally accessible fillet 23 of appropriatesealing material such as glass or fused metal which may be broken or cutaway to permit removal and replacement of the header whenever necessary.

The keep-alive electrodes l1 and I. are supported on the upper surfaceof the header 20 by means of relatively rigid lead-in conductors 25 and26 brought into the discharge device through the header. In orderhermetically to seal the lead-in connections, one may employ a quantityof glass such as is illustrated at 21, this being fused to'an eyelet 28of iron-nickelcobalt alloy or the like which is in turn welded orotherwise secured to the header.

In order to insulate the lead-in connections from the cathode material aportion of the sealing material may'extend above the level of thecathode surface as indicated, for example, at 30. Since the sealingmaterial is to a certain extent subject to erosion by the action of anare discharge, its exposed portion may be further protected by theprovision of a heat shield, for example, a sleeve 3| of metal, glass orquartz surrounding the same. Similar sleeves may be provided inconnection with all the electrode connections.

The keep-alive electrodes themselves are preferably constituted of aheat resisting material of high thermal emissivity such, for example, asgraphite, and may assume various configurations according to thecircumstances of thelr use. The electrodes l1 and 18, of which variousaspects are shown in Figs. 2, 3 and 5, comprise, in the caseillustrated, elongated graphite members of generally oblongcross-section. Where it is desired to use the holding-arc assembly withan alternating current power supply, the holding arc may be continuouslymaintained by employing two or more keep-alive electrodes adapted to beenergized during alternate periods. With a direct current supply onlyone such electrode is required.

In order to start the holding-arc during the initial functioning of thedischarge device, one may employ a make-alive electro such, for example,as a body of semi-conducting material 22 arranged in permanent contactwith the cathode. Such an electrode preferably has a resistivity betweenthe limits of from about 10- to about 10 ohms per centimeter cube andmay consist, for example, of a short tapered rod of silicon carbidewhich has been treated superficially with a solution of iron nitrateaccording to the procedure described and claimed in J. M. Cageapplication, Serial Number 130,760, filed March 13, 1937, and assignedto the same assignee as the present application. As iswell known in theart, the act of subjecting a make-alive electrode of the type specifiedto a positive potential during a period when one of the keep-aliveelectrodes restricted passage between such space.

is also positive with respect to the cathode II will cause an arcdischarge to such electrode to be initiated. A terminal connection forpermitting the make-alive electrode 33 to be energized is shown at 34.It is shown partly broken away so as not to obscure the remainingdetails of the assembly.

An exemplary circuit for obtaining proper cooperation between thekeep-alive and make-alive electrodes is shown diagrammatically inFig. 1. In the illustrated circuit there is provided a transformerhaving a tapped secondary 35 connecting with the various electrodes. Thekeep-alive electrodes are alternately rendered positive with re-- spectto the cathode I l by means of a symmetrical circuit arrangement inwhich the cathode is connected at 35 to the mid-point of a section ofthe transformer 35, the terminals of such section being connected to theelectrodes i1 and i3. Energization of the make-alive electrode 33 iscontrolled by a relay 36 which is so constructed as to be normallyclosed during periods of disuse of the discharge device. Under thesecircumstances application of potential to the transformer results inpassage of current to the electrode 33. This current. is caused to fiowonly in a desired direction (i. e. from the electrode 33 to the cathodeH) by the use of a rectifier 31. A current limiting resistor 33' isprovided in series with the make-alive electrode and the reactors 31' inseries with the keep-alive electrodes.

With the circuit illustrated, energization of the electrode 33 willresult in the immediate establishment of an arc discharge to thekeep-alive electrodes I! and II, which will thereafter conjointlymaintain the arc. Furthermore, once current starts to fiow in thecathode circuit, the relay 36 will act to open the connection to themake-alive electrode 33, thus maintaining it deenergized as long as theholding arc is in operation. If the holding arc is extinguished for anyreason the relay 33 will automatically close and the starting operationwill be repeated.

As previously stated, there is a tendency for the cathode spotmaintained by a holding-arc to wander freely over the cathode surfaceand even to run on to the walls of the discharge container. In order toavoid this type of operation and its attendant dimculties, my inventionprovides a shielding means adapted to confine the holding arc cathodespot to a restricted region of the cathode surface. Such meanspreferably comprises a structure which is adapted substantially tosegregate the holding arc space from the main discharge space except forthe provision of a Thus, with the particular electrode assembly shown,the shielding means consists of a hollow enclosing member extending froma point below the surface of the ca hode to and around the keepaliveelectrodes "and I8 and preferably also enclosing the make-aliveelectrode 33. Such an enclosing member (comprising parts to be describedhereinafter) may be supported in various ways and in the particulararrangement shown is mounted on relatively rigid supports 38 extendingfrom the upper surface of the header 20.

The shielding member should preferably be of a. refractory material, forexample, of metal or ceramic, which is adapted to withstand thedestructive action of an arc discharge. It may comrefractory insulatorsuch as quartz or alumina in view of the superior heat resistingqualities of these materials. I have found however, that if theinsulator extendsbelow the cathode surface so that it is frequently incontact with the cathode spot, there is some tendency for it to beeroded .by the action of the arc at the cathode spot. For

this reason, I prefer to construct the lower portion 43 of the shieldingmeans of a refractory metal, for example, of molybdenum or tungsten. Themetals last mentioned are particularly suitable in that they serve theadditional function of anchoring the cathode spot rather than permittingsuch spot to play freely over the entire enclosed portion of the cathodesurface.

In order that the cathode spot maintained by the holding arc may beeffective to initiate a discharge upon the application of a favorablepotential difierence to the main anode and cathode, at least somecommunication must be provided between the holding arc space and themain discharge space. In accordance with my invention such communicationis limited to a passage or passages of sufliciently small dimensions sothat the fiow therethrough of a current of the order of magnitude of theholding arc current will build up a substantial opposing voltage orvoltage drop. The significance of the foregoing statement may best beunderstood by consideration of a particular structure such as that shownin Figs. 2 and 3. In the arrangement there illustrated, the passagebetween the holding arc space and the main discharge space comprises aplurality of slots ll formed in the metal enclosure member 40. Theseslots ll preferably "extend normally from the cathode surface to a pointappreciably above such surface.

During the inactive period of the discharge device, that is when nopositive potential is applied between the main anode and the cathode,the holding arc cathode spot is readily retained within the regionenclosed by member 40. However, when the main anode l2 (Fig. 1) becomeshighly positive with respect to the cathode, a main discharge will beinitiated which in the first instance derives electrons from the cathodespotreferred to. As the main discharge current tends to increase, thelimited area of the slots 4| hinders the passage of current therethroughand produces a tendency for the cathode spot to be forced outside theenclosing member. This tendency is opposed, however, by the fact thatthe current demanded by the keep-alive electrodes themselves will,assuming a sufliciently small slot area, build up a relatively highvoltage drop in the slots if the cathode spot seeks to leave theenclosed region. This "constriction voltage drop" which is well-known inother connections, is comparable in many respects to the pressure droprequired to force a stream of liquid at high velocity through a.restricted tube or orifice.

- Under the influence of these conflicting forces the cathode spotfollows the only course left and divides. The component attributable tothe holding arc remains within the confines of the member ll whereas thecomponent attributable to the main discharge space may wander over thefree cathode area outside the enclosure. This latter component, however,extinguishes at the end of each positive half cycle of potential andmust be reignited from the holding arc at the beginning of the nextpositive half cycle. In this way the cathode spot is always reinitiatedat some point on the surface of the cathode II and is prevented frombecoming fixed to a destructible element of the discharge device such asthe wall of the enclosing envelope.

The slot area required to give the results stated in the foregoing isvariable with the magnitude of the holding arc current and must be lowenough with relation to the particular current employed so that theconstriction effect will prevent the portion of the cathode spotattributable to the holding are from leaving the shielded enclosure.This relationship does not lend itself to simple calculation but may bereadily determined experimentally. It will be noted that if the slotarea is too large, the "constriction voltage drop" may be inappreciableand will not prevent the spot from leaving the shield. On the otherhand, if the slot area is decreased sumciently this voltage drop willreach a substantial value, adequate to force a division of the spot sothat one component remains within the shield while the other suppliesthe demands of the load current. In general, this division will occurwhenever the voltage drop "needed to force through the slots a currentof the magnitude of the holding-arc current reaches a substantial 2fraction, say one-third to one-half, of the total voltage normallyrequired to maintain the holding-arc.

Referring again to the particular construction of Fig. 2 it has beenfound that with a moderate holding arc current, such as 5 to 'l ampereseight slots provide too little shielding where each slot it .045 inchwide and extends as much as inch above the cathode surface or where eachslot is .025 inch wide and extends inch above the cathode surface. Theresult is that not enough constriction occurs and the main anodeattempts to get too much of its current from under the cup, resulting inexcessive'heating. Further, the constriction is insumcient to force a,

division of the cathode spot so that the spot frequently leaves theshield during the conduction period. Under these conditions, the cathodespot is sometimes recaptured when the load current ceases, but morefrequently extinguishes. How'- ever, the .045 inch slots have been foundto be satisfactory if limited to about inch length while .025 inch slotsprove satisfactory if limited to about inch length. It should beunderstood, however, that these exemplary figures refer to a particularconstruction of the shielding enclosure and to a particular value of theholding-arc current. Widely different dimensions may prove suitable withenclosures of diflerent form and materials and with other electrodearrangements.

The precise type of shielding enclosure shown in Figs. 2 and 3 is in noway essential to the purposes of the invention, and in Figs. 6 and 7 Ihave shown one alternative form which the shield may take. In this case,as in that previously described, the upper part of the enclosure III ispreferably of an insulating material such as quartz alumina or glasswhile the lower part M is of a refractory metal such as molybdenumhaving slots 52 therein (Fig. 7) However, the lower as part andparticularly that portion in contact with the cathode surface has arelatively constricted configuration for a purpose to be stated in thefollowing,

The nature of this configuration is most clearly shown in Fig. 7 whereinit appears that the metallic shield portion ll comprises a substantiallycontinuous enclosure having a. long and narrow outline. It is aparticular advantage of this construction that the holding-arc cathodespot, even though it may occasionally escape from anchortungsten, havingdepending therefrom a semiconducting make-alive electrode It in contactwith the cathode surface. During the initial starting period of thedevice, a positive potential is impressed on the member it through theleadin conductor ll. The current flowing through the semi-conductingelectrode member I will initiate an arc, the positive terminal of "whichwill be immediately transferred to the conducting member I or to someportion thereof. Due to the high resistance of the member I4, it will beeffectively shunted by the holding-arc and will play no further part inthe operation of the device. The holding-arc itself may be maintainedbetween the member If and the surface of the cathode material by themaintenance of a direct current potential between them.

The shape and character of the constricting passage may also be variedand in Fig. 8 I have shown one form which such variation may take. Inthis case, the shielding enclosure is formed by an inverted cup-likemember ll, suitably of quartz, having a flattened rim ll arrangedparallel to the surface of the cathode 82. Within the shielding memberll, there are provided keepalive electrodes 84 and ll and a make-aliveelectrode 66 corresponding in essential particulars to the similarelements previously described. The narrow free space or passage I(preferably of the order of magnitude of it of an inch in height)existing beneath the rim 8|, serves by constriction to maintain theholding arc cathode spot within-the region falling under the member llin accordance with the principles previously explained. It is clear,however, that if the passage 63 is defined only by the cathode surfaceand by theopposed surface of the rim OI, slight variations in thecathode level will produce relatively great variations in the operationof the holdin arc assembly. For this reason in order satisfactorily toemploy the construction of Fig. 8 with a liquid cathode such as mercury,particularly in high current rectii'iers, I consider it necessary toprovide means for stabilizing the cathode at the constricted region.Such means may include, for example, an annular member 88 constituted ofa porous wick-like substance adapted to be wet by the cathode material.Such a substance, suitable for use with mercury, may comprise in oneinstance, a slntered mass of coherent particles of carburized molybdenumsuch as is described and claimed in U. S. Letters Patent No. 2,128,861

granted August 30, 1938 toLewi Tonks and assigned to the same assigneeas the present invention. The annular member 68 may project slightlyaboye the normal surface of the cathode, dependence being placed uponcapillarity to maintain the upper surface of the member covered withcathode material so that it operates stabilizing member I will besubstantially constant and independent of variations in the oathodelevel. Still further means for preventing excessive agitation or wavemotion of the cathode material in the region directly surrounding theholding-arc assembly is provided in the form of an annular baille l9,suitably of metal, extending around the outer periphery of the member 99and arranged slightly below the cathode surface. (It should also benoted that in many cases a' bailie of this kind may advantageously beassociated with holding-arc shielding other than the'particularshielding means illustrated in Fig. 8.) Mounting brackets II serve tosupport the member 99 from the upper surface of the baIlie I9.

In Fig. 9 I have shown a further modification which differs from thearrangements previously described mainly with respect to the meansdisclosed for initially starting the holding arc. In this figure, suchmeans is illustrated in the'form of a sparking electrode I9 although itmay alternatively comprise a capacity make-alive device such as isshown, for example, in Patent 1,119,572 to Von Recklinghausen. Duringthe initial starting of the device, a high potential may be impressedbetween the cathode I9 and the sparking electrode 15, for example, bymeans of a socalled ignition transformer 11. discharge thus producedwill be eifective to initiate a holding-arc to the keep-alive electrodesI9 and 19. These latter electrodes are energized by means of atransformer 99 which has a center tap connection to the cathode l9 andwhich is connected at its terminals to the keep-alive electrodes throughstabilizing reactors II. A series relay 82 in the holding arc circuitacts to break the connections to the starting electrode! as soon as theholding-arc is in progress.

Another feature shown in connection with Fig. 9 comprises a baiiie l4,suitably of metal, interposed between the keep-alive electrodes 19 and19. Such a baiile may advantageously be employed with any holding-arcassembly in which a plurality of keep-alive electrodes are employed,

and serves to prevent cross currents between the various electrodesduring periods when a diiler-w ence of potential exists between them.

The various embodiments of the invention so far described have beenprimarily applicable in facilitating or assuring the initiation of adischarge upon the application of a-favorable potential between the maindischarge electrodes. The devices of ;my invention may also be employedin connection with control means to determine the occurrence ornon-occurrence of a discharge to a principal anode upon theestablishment of conditions favorable to such discharge, or to vary thetime of discharge initiation in a desired manner. In order to understandthis aspect of the invention, it is desirable to refer briefly to someof the principles which govern the occurrence of a discharge in a vaporarc device of the type herein under consideration.

When a positive potential is impressed between an anode and a pool-typecathode on which a cathode spot exists, the occurrence. of an arcdischarge to such anode .depends largely upon the presence in thedischarge space of a supply of electrons sufficient to produceionization of the vapor existing in the space. It is the usual functionof a holding-arc and its cathode spot to provide and maintain such anelectron supply in the main discharge space. However, in connection withshielded holding-arc assemblies such as those herein described this isnot necessarily true. on the contrary, with such .The sparking Referringparticularly to Fig. 19, I have shown 'a holding-arc assembly comprisinga single keepalive electrode 90, (to be energized from a direct currentsource). and a make-alive electrode 9|. These electrodes are enclosedwithin a hollow shielding member comprising an upper insulating portion99 and a lower slotted metal portion 99. In addition to the elementsalready described, there is provided an electrostatic control member orelectrode in the form of a conducting band 99, for example, of metalmounted on the outer surface of the insulating member 93 in operativerelation to the slots in the metal member 94.

Circuit connections for a device of the type described in the foregoingare illustrated diagrammatically in Fig. 11 which also illustratesschematically a complete discharge device comprising a cathode 91, ananode 99 and an enclosing envelope I99. The make-alive electrode 9| maybe energized for initially starting the holding-arc by means of a directcurrent source of potential I92, controlled by a switch I93; while theholding-arc may be maintained by a connection between such source andthe keep-alive electrode 99. As soon as the holding-arc is started, theoccurrence of a main discharge will depend in part upon the applicationof afavorabie potential diflerence between the anode 99 and the cathode91 and in part on the potentiaiapplied to the control member 99. In thisconnection it will be understood that if the member 96 isstrongly biasednegatively, it may be made impossible for the positive field of theanode 99 to draw electrons from within the shielded space in sufllcientquantity to initiate a main discharge. The magnitude of the negativepotential which must be applied to the member 96 to accomplish thisresult will depend in part upon the geometry of the discharge device andof the shielding structure and in part upon the magnitude of thepositive potential tobe applied to the anode 99.

Assuming that the control electrode 99 is sufficiently negatively biasedby means of a battery I99 so that the occurrence of a main dischargewill be normally impossible, such a discharge can nevertheless be causedto occur by additionally applying to the control memberanother-potential of such nature as to overcome its bias. Such apotential may be provided, for example, by means of an alternatingcurrent transformer I91 having its primary connected to a potentialsource I98 and its secondary'ln circuit with the control electrodethrough a series resistor I99. Interposed between the source and thetransformer there may be provided a phase-shifting means, illustrateddiagrammatically at which is adapted to control the phase relationbetween the control potential applied to the electrode 96 and the maindischarge potential applied between the anode 99 and the cathode 98. Byproperly selecting this phase relationship as by adjustment of the phasecontrol means H9,

the main discharge may be made to occur cyclically and may also belimited as to duration to a selected fraction of each positive halfcycle of the main potential. Other means of control will of course occurto those skilled in the art.

It is important to note that the control member of the combination justdescribed acts only on the holding arc and exerts no direct influence onthe main discharge current. It therefore has no tendency to increase themain arc drop, in which respect it differs fundamentally fromthe variousgrid structures and other electrostatic control means which havepreviously been employed in connection with the regulation of pooltypedischarge devices.

In order somewhat to decrease the potential which must be applied tothecontrol member for eifective control operation and to decrease thenecessary power consumption of the control circuit, I may also providemeans for additionally shielding the holding-arc cathode spot from thefield of the main positive electrode or anode. One means is shown inFig. 12 wherein the holding-arc assembly itself closely resembles thatalready described in Fig. 2. In this figure, electrostatic shieldingmeans in the form of a pair of cylindrical grids III and H4 are providedin a region closely surrounding the passage existing between the holdingarc space and the main discharge space. These members are preferably ofsufilcient extent to encompass also the control member III. They areshown as being in contact with the bottom of the discharge envelope .andtherefore with the cathode material (indicated in dotted outline at III)but may alternatively be insulated therefrom and charged to differentpotentials. Under the conditions illustrated, they tend to maintain inthe region surrounding the holding-arc shielding structure anequipotential zone having the same potential as the cathode itself. Itis thus made relatively difilcult for the positive field of the mainanode to affect in any way conditions within the holding arc shieldingstructure. For this reason, the occurrence of a main discharge is moredirectly under the control of the control member III and may beregulated by the use of a control potential of relatively low value,applied, for example, by means of a transformer I It in connectionwith aphase-shifting device Iii.

A still further variation of structure in connection with a controllabledischarge-initiating device is shown in Figs. 14-16. In the arrangementthere illustrated, the electrode structure involved comprises a singlekeep-alive electrode I30 and a make-alive electrode Ill suitablysupported above the surface of a pool-type cathode I32. A shieldingmember is provided in the form of an inverted cup-like member I 34, ofquartz or the like, which encloses the keep-alive electrode I30 andwhich is Joined at its lower end to a body of refractory material I36,suitably of graphite or alumina. The member I38 is of such configurationthat the cathode material within the shielding enclosure is confined toa relatively constricted well I31 (Fig. 16) communicating with aradially extending narrow channel I". The channel itself issubstantially closed by means of a pair of barriers I40 and I,preferably of a refractory metal such as molybdenum and each having oneor more slots therein. With this arrangement, the cathode spot withinthe shielded enclosure will be confined to a very limited portion of thecathode surface. Ordinarily it will be anchored to the inner face of thebarrier I40 so as to be close to a slotted opening I43 provided in suchbarrier. In some cases I have found it advantageous to. assist suchanchoring by the use of means (not shown) for producing a magnetic fieldtransverse to the holding arc and in a direction suitable for forcingthe cathode spot against the barrier, thereby to localize the same. Thisfeature is fully described and claimed in my copending applicationSerial No. 254,156, filed February 2, 1939. By virtue of such anarrangement, highly dependable results with respect to firing of thedischarge device may be obtained whenever conditions enternal to theshielded enclosure are favorable to such firing.

In order to shield the holding arc cathode spot most effectively fromthe field of the main anode, the second barrier I has an opening I45which is offset from the opening I43 in the barrier I40 (see Figs. 15and 16). In order to control the discharge initiation, an electrostaticcontrol electrode I46 is provided in operative relation to the channelI". This control electrode may be energized in a desired manner, as haspreviously been explained and when properly charged will be effective toprevent firing of the discharge device in spite of the occurrence of afavorable potential difference between the main discharge electrodes.when diiferently charged, on the other hand, it may permit such firing.It will be understood that the particular arrangement of the electrodeI46 shown in Fig. 14 is not essential. In some cases the control membermay advantageously be placed between the barriers I40 and I. or evenwithin the shielded enclosure. With the last mentioned arrangement, itcan be made to aifect the magnitude of the holding-arc current and inthis way to control the number of electrons supplied to the maindischarge space.

It will be seen from the foregoing that I have provided in connectionwith a pool-type discharge device an improved discharge-initiatingdevice by means of which the cathode spot may be positively confined tothe cathode surface. Furthermore such confinement is accomplishedwithout the use of an electrode member in continuous contact with thecathode surface so that the limitations on life ordinarily associatedwith such electrodes are not involved. 4

By shielding the holding-arc assembly in the manner specified, I alsoobtain the result that the operation of the holding arc is substantiallyindependent of events occurring in the main discharge space. Inparticular, there is little or no tendency for the holding arc to beextinguished by arc-back or excessive forward currents occurring betweenthe main electrodes. For this reason, an extremely low holding arccurrent, of the order of a few amperes, may be employed satisfactorily.Also, because of the inherent reliability of my improved system, verylittle auxiliary and supervisory equipment is required.

While I have in the foregoing exemplified my invention by reference toparticular constructions, materials, and dimensions, it will be understood that any or all of these may be varied substantially by thoseskilled in the art without departure from the invention. I, therefore,aim in the appended claims to cover all modifications which come withinthe true spirit and scope of the foregoing disclosure.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

1. A discharge device comprising an anode and a pool-type cathode havinga discharge space therebetween and means for facilitating the imin agenerally normal direction with respect to the application of afavorable difference of potenclosing the keep-alive electrode except forthe provision of one or more narrow slots in the member at the cathodesurface, such slots extending the cathode surface.

2. A discharge device comprising an anode and a pool-type cathode havinga discharge space therebetween and means for facilitating the initiationof a discharge through the space upon the application of a favorabledifference of potential between said anode and cathode, said meanscomprising a keep-alive electrode in operative relation to the cathodesurface for maintaining a holding arc thereto, and a hollow shieldingmember extending from a point below the cathode surface to and aroundthe keep-alive electrode, said member substantially separating theholding-arc from the main discharge space except for the provision ofone or more openings through the member at the cathode surface, theportion of said shielding member which is in contact with the cathodesurface being of materially.

smaller transverse dimensions than the upper portion of the member so asto confine the hold-' ing-arc cathode spot-to a region which is closelyadjacent to said openings.

3. A discharge device comprising means providing a principal dischargespace, means including a pool-type cathode affording terminals for adischarge passing through said space, a keepalive electrode in operativerelation to the cath-" ode surface for maintaining a holding-arcthereto, a shielding member extending from the cathode surface to andaround the keep-alive electrode, said member substantially separatingthe discharge path of the holding-arc from the main discharge spaceexcept for the provision of one or more restricted openings between themat the cathode surface, and means including a body. of acathode-spot-anchoring material in contact with the cathode surface foranchoring the cathode spot of the holding arc in the vicinity of saidopenings during the use of the device.

4. A discharge device comprising means forming a principal dischargespace, means including a pool-type cathode providing terminals for aprincipal discharge passing through such space, and means forfacilitating the initiation of a discharge through the space uponapplication of a favorable potential difference between such terminals,said last-named means including an electrode cooperating with thecathode for maintaining a holding-arc and a hollow member extending froma point below the cathode surface to and around the keep-aliveelectrode, the portion of said hollow member more remote from thecathode comprising a refractory insulating material and the portion incontact with the cathode material comprising a refractory metal bodyprovided with one or more restricted openings.

5. A discharge device comprising an anode and a liquid metal cathodehaving a discharge space therebetween and means for facilitating theinitiation of a discharge through said space upon the application of afavorable difference of potential between said anode and cathode, saidmeans comprising a keep-alive electrode in operative relation-to thecathode surface, and a hollow arating the keep-alive electrode from themain discharge space except for the provision of a narrow passage tosaid space between the lower edge of the shielding member and thecathode surface, and means for stabilizing the cathode surface tomaintain the dimensions of said passage substantially constant duringthe use of the device.

6. A discharge device according toclaim in which the cathode surfacestabilizing means includes a body of a wick-like substance capable ofbeing wet by the cathode material and adapted to anchor the cathodespot.

' 7. A discharge device comprising means forming a principal dischargespace, means including a pool-type cathode providing terminals for adischarge passing through said space, a keep-alive electrode cooperatingwith the cathode surface for maintaining a holding-arc adapted tofacilitate initiation of a discharge between said terminals, and meansfor controlling the initiation of such a discharge, said last-namedmeans including a hollow enclosure substantially segregating thedischarge space of the holding-arc the initiation of a main dischargethrough said space upon the application of a favorable difference ofpotential between the anode and cathode, said means including one ormore keep-alive electrodes effective to maintain a continuousholding-arc to the cathode surface throughout the normal operation ofthe device, and a hollow shielding-member of constant potentialextending from a point below the cathode surface to and around thekeep-alive electrodes, said shielding member substantially completelyenclosing the keep-alive electrodes except for the provision in themember of one or more restricted openings adjacent to the cathodesurface. the openings being of such dimensions as to prevent the holdingarc cathode spot from escaping the shielded enclosure, whilenevertheless permitting the ignition of the main discharge therefrom.

9. A discharge device comprising an anode and a pool-type cathodeproviding terminals for a main discharge, a keep-alive electrodecooperating with the cathode surface for maintaining a continuousholding-arc thereto throughout the normal operation of the device. ahollow shielding member extending from a point below the oathode surfaceup to and around the keep-alive electrode and completely enclosing thesame except for the provision of a restricted passage through the memberadjacent to the cathode surface, said passage being .of such dimensionsas to prevent the holding-arc cathode spot from escaping the shieldedenclosure while nevertheless permitting the ignition of the maindischarge therefrom, and an electrostatic control electrode arranged inoperative relation to said passage for controlling initiation of themain discharge.

, LAWRENCE :o. mms.

